The present invention relates to a high-pressure discharge lamp. Specifically, the present invention relates to a high-pressure discharge lamp preferably used for lighting at such usages for high ceilings, stores, and streets.
Conventionally, high-pressure discharge lamps for such usage as high ceilings, stores, and streets comprise a light emission tube made of quartz glass or ceramic, an outer tube, and wire frames made of a conductive material for supporting the light emission tube at the outer tube (for example, refer to U.S. Pat. No. 6,326,721). Since the light emission tube of this kind of high-pressure discharge lamp is heated to a very high temperature during lighting, relieving the thermal stress generated in the light emission tube is critical for preventing the breakage of the light emission tube. U.S. Pat. No. 6,326,721 discloses a structure where the stress due to the thermal expansion of the light emission tube during lighting is relieved by a coil provided at one end of the wire frame.
Further, there are other prior art arrangements for similarly preventing the breakage of the light emission tube. In such prior arts, a compressive stress latently exerts the material of the light emission tube in advance in order to relieve the tensile stress to be generated on the surface of the light emission tube during lighting (for example, refer to Japan Patent Application Laid-open Publication No. 2-301957 and Japan Patent Application Laid-open Publication No. 60-225159). These prior art references intend to cancel the tensile stress generated during lighting by the compressive stress latently exerted, thereby preventing the breakage of the light emission tube.
The lighting conditions required for high-pressure discharge lamps have been changing recently. The conditions are broadly classified into two conditions. As a first condition, under the circumstance where these high-pressure discharge lamps, in particular, metal halide lamps, are required to have higher efficiency, the operation pressure in the light emission tube is required to be increased from a conventional pressure of several atms (about 5 to 9 atms) to a pressure of ten-odd atms (about 10 to 15 atms) to improve lighting efficiency. Several methods are available to raise the operation pressure. For example, a general method for increasing operation pressure is to make the light emission size smaller for increasing a load applied to a tube wall and raise the temperature of the light emission tube higher than a conventional temperature so as to accelerate evaporation of sealed metals. Another condition relates to the lighting posture of the lamp. Although the lamp has been used in a vertical lighting posture relatively frequently, the use of the lamp in a horizontal lighting posture is increasing in view of the design of lighting apparatuses, in particular, the design for attaining space-saving.
However, the above-mentioned prior art devices are all intended to relieve the thermal stress generated in the light emission tube during lighting at an operation pressure of several atms in a vertical lighting posture. The above-mentioned prior art devices do not provide countermeasures against the thermal stress generated in the light emission tube under at a high operation pressure of ten-odd atms in a horizontal lighting posture.